Positive or negative charged will induce a dipole on the neutral charge, causing it to attract to either charge. Adding protons to an atom causes positive charge and adding electrons to an atom adds negative charge. Your hair and the gallon are naturally charged, so they induce a dipole in each other and cause themselves to mutually attract. Inducing a dipole is when a neutral charge has some of its positive or negative charges dragged to one side creating net positive charge on one side and net negative charge on the other.
